1. Field of the Invention
The present invention relates to a method for driving a plasma display panel (PDP).
There is a task of improving light emission efficiency for a display using a plasma display panel. It is desired to realize a brighter display with less power consumption. The light emission efficiency depends not only on a cell structure but also on a driving method.
2. Description of the Related Art
A driving method of an AC type plasma display panel utilizes wall voltage for a display. The wall voltage is generated when a dielectric layer that covers a pair of display electrodes is charged. Wall voltages of cells in which display discharge is to be generated among cells within a screen are set higher than wall voltages of other cells, and then an appropriate display pulse (also called a sustain pulse) is applied to every cell at one time. When the display pulse is applied, a drive voltage is added to the wall voltage. The display discharge is generated only in cells that have sum voltage of the drive voltage and the wall voltage exceeding a discharge start voltage. Light emission by the display discharge is called “lighting”. Utilizing the wall voltage, only cells to be lighted can be lighted selectively.
The display pulse is applied plural times that is set to the number corresponding to brightness of the display so that a polarity of the drive voltage is reversed every time. An application period is approximately a few microseconds, so that the light emission is observed to be continuous. When display discharge is generated by the first application, wall charge on the dielectric layer is erased once, and regeneration of wall charge is started promptly. A polarity of the regenerated wall charge is opposite to the previous one. When the wall charge is reformed, a cell voltage between display electrodes drops so that the display discharge ends. The end of discharge means that discharge current flowing in the display electrode becomes substantially 0 (zero). The application of the drive voltage to the cell continues until the trailing edge of the display pulse after the display discharge ends. Therefore, the space charge is attracted to the dielectric layer in an electrostatic manner, and reformation of the wall charge is progressed. Each of the display pulses has a role of generating display discharge and reforming an appropriate quantity of wall charge.
In general, the display pulse has a rectangular waveform. In other words, a usual driving circuit is constituted to output a rectangular waveform. In a design of the driving circuit, amplitude of the display pulse, i.e., a sustaining voltage Vs having a rectangular waveform is determined to be a value within a permissible range that is determined on the basis of discharge characteristics of the plasma display panel. If the sustaining voltage Vs is set to a value higher than the maximum value Vsmax that is nearly the discharge start voltage Vf, discharge may be generated also in a cell that is not to be lighted. In addition, if the sustaining voltage Vs is set to a value lower than the minimum sustaining voltage Vsmin that is a lower limit value, the wall charge cannot be reformed sufficiently, resulting in unstable repeat of lighting.
A typical driving method in which a rectangular display pulse is applied cannot improve both luminance and light emission efficiency. When the amplitude of the display pulse is increased within a permissible range, intensity of the display discharge can be enlarged so that the light emission luminance can be improved. However, the attempt to increase the light emission luminance may cause increase of power consumption and drop of the light emission efficiency. A solution of this problem is described in Japanese unexamined patent publication No. 10-333635, in which a display pulse is applied that has a step-like waveform with a leading edge having locally large amplitude.
In addition, Japanese unexamined patent publication No. 52-150941 discloses another waveform of the display pulse that has a step-like waveform in which the amplitude increases between a leading edge and a trailing edge. This step-like waveform has an advantage that can generate discharge at a low voltage and form an adequate quantity of wall charge.
There is a problem in the conventional driving method, which is that electric power is consumed wastefully when the number of cells to be lighted is small regardless that the display pulse waveform is either the rectangular waveform or the step-like waveform. When the number of cells to be lighted is small, discharge current in the entire screen and the voltage drop in the power source are smaller than in the case where the number of cells to be lighted is large. Namely, the minimum sustaining voltage Vsmin is higher as the number of cells to be lighted is larger. In contrast, the appropriate sustaining voltage Vs is relatively low when the number of cells to be lighted is small. However, when designing a display pulse, it is important to determine the amplitude of the display pulse in consideration of a voltage drop when the number of cells to be lighted is the maximum, i.e., all cells are lighted, so that a correct display is realized regardless of the number of cells to be lighted. As explained above, if the amplitude of the display pulse is determined on the basis of the drive when the number of cells to be lighted is large, an excessive voltage may be applied to cells to form excessive wall charge when the number of cells to be lighted is small. As a result, a loss of electric power will be increased, and the light emission efficiency will drop.